Audit Age Log Limit

By default, audit logging is configured to store audit log entries for 90 days.

After 90 days, the audit log entry is cycled. You can change the audit log age limit using the Set-Mailbox cmdlet with the AuditLogAgeLimit parameter.

You can specify the number of days, hours, minutes, and seconds to retain audit log entries.

Logs need not be retained for a long time (more than a few days), since File Access Manager offloads the data from the exchange.

It is not recommended to retain an audit for a long time, since doing so expands the Exchange DB.

The following site provides more information:

https://technet.microsoft.com/en-us/library/bb123981%28v=exchg.150%29.aspx